Basin-wide integrated volume transports in an eddy-filled ocean

The temporal evolution of the strength of the Atlantic Meridional
Overturning Circulation (AMOC) in the subtropical North Atlantic is
affected by both remotely forced, basin-scale meridionally coherent,
climate relevant transport anomalies, such as changes in high-latitude
deep water formation rates, and locally forced transport anomalies,
such as eddies or Rossby waves, possibly associated with small
meridional coherence scales, which can be considered as noise. The
focus of this paper is on the extent to which local eddies and Rossby
waves when impinging on the western boundary of the Atlantic affect
the temporal variability of the AMOC at 26.5N. Continuous estimates
of the AMOC at this latitude have been made since April 2004 by
combining the Florida Current, Ekman, and midocean transports with the
latter obtained from continuous density measurements between the
coasts of the Bahamas and Morocco, representing, respectively, the
western and eastern boundaries of the Atlantic at this latitude.
Within 100 km of the western boundary there is a threefold decrease in
sea surface height variability toward the boundary, observed in both
dynamic heights from in situ density measurements and altimetric
heights. As a consequence, the basinwide zonally integrated upper
midocean transport shallower than 1000 m - as observed continuously
between April 2004 and October 2006 - varies by only 3.0 Sv RMS.
Instead, upper midocean transports integrated from western boundary
stations 16, 40, and 500 km offshore to the eastern boundary vary by
3.6, 6.0, and 10.7 Sv RMS, respectively.
The reduction in eddy energy toward the western boundary is reproduced
in a nonlinear reduced-gravity model suggesting that boundary-trapped
waves may account for the observed decline in variability in the
coastal zone because they provide a mechanism for the fast equatorward
export of transport anomalies associated with eddies impinging on the
western boundary. An analytical model of linear Rossby waves suggests a
simple scaling for the reduction in thermocline thickness variability
toward the boundary. Physically, the reduction in amplitude is
understood as along-boundary pressure gradients accelerating the fluid
and rapidly propagating pressure anomalies along the boundary. The
results suggest that the local eddy field does not dominate upper
midocean transport or AMOC variability at 26.5N on interannual to
decadal time scales.